Add the drained lentils in a large Dutch oven and cover with 4 times their volume of cold water.
Cover for 20 minutes after reaching boiling point: first 10 minutes on high heat and then, 10 on medium to high heat.
Add the rice and ¾ cup (200 ml) of boiling water. Mix well and cook uncovered for 15 minutes over medium heat, stirring regularly.
If necessary, add a little boiling water if the rice absorbs the water quickly.
A few minutes before the end of cooking, season with salt and pepper and mix well.
While cooking the lentils and rice, heat olive oil in a large skillet and fry the sliced onions until the edges curl and turn golden brown.
When the 2 preparations are ready, pour the hot oil (without the onion) into the mujaddara pot, and mix well.
Arrange on a plate.Top with the fried onions.